cache/filecache: Add a cache prune func

Fixes #5439

cache/filecache/filecache_pruner.go

@@ -0,0 +1,80 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package filecache
+
+import (
+	"io"
+	"os"
+
+	"github.com/pkg/errors"
+	"github.com/spf13/afero"
+)
+
+// Prune removes expired and unused items from this cache.
+// The last one requires a full build so the cache usage can be tracked.
+// Note that we operate directly on the filesystem here, so this is not
+// thread safe.
+func (c Caches) Prune() (int, error) {
+	counter := 0
+	for k, cache := range c {
+		err := afero.Walk(cache.Fs, "", func(name string, info os.FileInfo, err error) error {
+			if info == nil {
+				return nil
+			}
+
+			name = cleanID(name)
+
+			if info.IsDir() {
+				f, err := cache.Fs.Open(name)
+				if err != nil {
+					// This cache dir may not exist.
+					return nil
+				}
+				defer f.Close()
+				_, err = f.Readdirnames(1)
+				if err == io.EOF {
+					// Empty dir.
+					return cache.Fs.Remove(name)
+				}
+
+				return nil
+			}
+
+			shouldRemove := cache.isExpired(info.ModTime())
+
+			if !shouldRemove && len(cache.nlocker.seen) > 0 {
+				// Remove it if it's not been touched/used in the last build.
+				_, seen := cache.nlocker.seen[name]
+				shouldRemove = !seen
+			}
+
+			if shouldRemove {
+				err := cache.Fs.Remove(name)
+				if err == nil {
+					counter++
+				}
+				return err
+			}
+
+			return nil
+		})
+
+		if err != nil {
+			return counter, errors.Wrapf(err, "failed to prune cache %q", k)
+		}
+
+	}
+
+	return counter, nil
+}